The present invention relates to a SCR catalyst system as well as to a method for operating said SCR catalyst system. The invention further relates to a computer program which carries out all of the steps of the method according to the invention if said program is executed on a computer or in a control device. Finally the invention relates to a computer program product with a program code, which is stored on a machine-readable carrier, for carrying out the method according to the invention if the program is executed on a computer or in a control device.
In order to comply with the increasingly stringent emission standard legislation (Euro 6, Tier 2 bin 5 and further emission guidelines), it is necessary to reduce nitrogen oxides or respectively nitric oxides (NOx) in the exhaust gas of internal combustion engines, in particular diesel engines. To this end, it is known to dispose a SCR catalyst (Selective Catalytic Reduction) in the exhaust gas region of internal combustion engines, said catalyst reducing the nitrogen oxides contained in the exhaust gas of the combustion engine to nitrogen in the presence of a reducing agent. In so doing, the proportion of nitrogen oxides in the exhaust gas can be substantially reduced. During the course of the reduction of the nitrogen oxides, Ammonia (NH3) is required that is added to the exhaust gas. For this purpose, NH3 reagents or rather reagents which split of NH3 are metered into the exhaust gas tract. In this case, an aqueous urea solution (UWS=urea-water solution) is used as a rule, which is injected into the exhaust gas tract upstream of the SCR catalyst. Ammonia forms from this solution which acts as the reducing agent. A 32.5% aqueous urea solution can be commercially obtained under the registered trade mark AdBlue®. In order to achieve high conversion rates of the nitrogen oxides to be reduced in a SCR catalyst system, the SCR catalyst has to be operated such that said catalyst is constantly filled to a certain level with the reducing agent. The German patent specification DE 10 2004 031 624 A1 describes, for example, how such a process control is set up for a SCR catalyst system on the basis of the ammonia fill level. The French patent specification FR 2 872 544 A1 describes a temperature-dependent, target filling level specification.
The SCR catalyst is generally implemented as a single component. Applications are however known in which the catalyst volume is, for example, divided into two housings for reasons of available space. In these applications, the two catalysts are nevertheless treated to the greatest extent as a single catalyst in terms of process control. Such SCR catalysts are in fact modeled in a partially location-discrete manner, for example in a plurality of recordings. These items of information are however bundled for the purpose of regulating the fill level, calculating the dosing quantities and the like; and in the end the method proceeds as in the case of a system having a single catalyst.